1. Field of the Invention
The present invention relates in general to the elimination of odors, and more particularly, to the process for removing odor-causing organic compounds from a liquid or gaseous medium by contact of the medium with a crystalline molecular sieve having certain critical physical and chemical properties hereinafter elucidated. The invention also relates to molecular sieve compositions suitable for use in the process.
2. Description of the Prior Art
Some substances are odorous, others are not. Just why this should be has been investigated many times, but to date remains an unanswered question, although many theories have been advanced. In accordance with the physical theory, odors are the interaction of high frequency radiations emitted by odorous substances with pigment granules in the olfactory receptors. The radiations are derived from the characteristic molecular vibration of each specific odorous substance, and distinctions between the various odors are made on the basis of the differences in wave lengths of the emitted radiations. In a chemical theory, it is the reactivity of functional portions of molecular species with olfactory receptors which is perceived by the brain as odor. It has more recently been speculated that the energy of the precise stimulus of odor receptors is derived electrochemically from an odorous material, more particularly from a modification of the bonding angles of a molecular species upon solution in the olfactory mucosa and a consequent alteration or unbalance of the electrostatic fields detected by the olfactory receptors. This theory could explain why multiple substances of quite different chemical structure may have the same or similar odors, and also why a mixture of two or more odorous substances may be perceived as having no odor.
Regardless of the reasons why certain substances exhibit odors, a vast number of means for eliminating those odors have been proposed. Generally, these processes can be placed in one of two categories, i.e., (a) odor masking in which a substance of strong yet relatively pleasant odor is introduced into the zone containing the substance of a less pleasant odor with the intent of overburdening the olfactory receptors with the dominant pleasant odor; or (b) sequestering the undesired odorous substance in a non-volatile form either by chemical reaction, adsorption or absorption on a sorbent exhibiting a sorptive preference for the odorous substance. It is the latter category which is of greater relevance to the present invention, although it is not known whether the mechanism involved is physical adsorption or chemisorption, or both.
By far the most commonly employed of the solid adsorbents is activated charcoal or active carbon, although silica gel, activated alumina, kieselguhr, fullers earth and other clay minerals and zeolites, alone or in combination, have also been proposed as odor "adsorbents". A mixture of silica gel, activated carbon and a zeolite is disclosed in U.S. Pat. No. 4,604,110 as the adsorbent for an air filter element for removing odors from indoor air. A mixture of alfalfa, bentonite clay and a binder is proposed in U.S. Pat. No. 3,789,797 for adsorbing and neutralizing odors of animal waste matter. In U.S. Pat. No. 4,437,429, the use of a hydrated zeolite in admixture with clay is proposed as being particularly useful for the control of odors from pet litter, it being observed that the use of zeolites by themselves as litter material has generally been unsuccessful due to their poor water adsorption properties as compared with clays. For the adsorption of certain odors from animal litter using a mixture of clays and zeolites, it is proposed in U.S. Pat. No. 4,437,429 that the zeolite constituent not only be employed in its hydrated state, but also that the water of hydration be the original water of hydration. It is said to be not sufficient that water is added to a previously heat-treated zeolite from which the original water of hydration was driven off.
In general, when zeolites have been utilized for odor suppression, the preferred species have been those with a low framework Si/Al ratio and a high degree of adsorption capacity for water or other highly polar molecules such as ammonia or hydrogen sulfide. The disclosure of the aforesaid U.S. Pat. No. 4,437,429 is somewhat remarkable for its specific disclosure of many of the class of so-called high silica zeolites which are synthesized using organic templating agents such as the tetraalkylammonium ions. These include ZSM-5, ZSM-11, ZSM-12 and ZSM-23. The preferred zeolite species is the natural mineral clinoptilolite, however, which has a nominal framework SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of about ten. It has also been proposed, U.S. Pat. No. 4,648,977, to use high-silica molecular sieves, including the silica polymorph silicalite, to adsorb toxic organic materials, including mercaptans, from aqueous media for water purification purposes.